(a) Field of the Invention
The present invention relates to a hydraulic control system, and more particularly, to a hydraulic control system for a four-speed automatic transmission in which a number of pressure control valves, solenoid valves and fail-safe valves is reduced.
(b) Description of the Related Art
Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction elements actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls pressure generated by an oil pump, to change shift ratios of the planetary gearset.
The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift selector to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the oil pump to each valve and actuator.
FIG. 4 shows a hydraulic circuit diagram of a conventional hydraulic control system. The hydraulic control system includes an underdrive clutch UD-C, an overdrive clutch OD-C, a reverse clutch REV-C, a low-reverse brake LR-B, and a second brake 2NB-B. These elements are controlled by hydraulic pressure and perform control to realize four forward speeds and one reverse speed.
Solenoid valves control hydraulic pressure supplied to a plurality of valves, which, in turn, control hydraulic pressure supplied to the friction elements. There are a total of four solenoid valves, one less than the number of friction elements, provided in the hydraulic control system. These include a low-reverse solenoid valve LR-S, a second solenoid valve 2ND-S, an underdrive solenoid valve UD-S and an overdrive solenoid valve OD-S. Valves controlled by the solenoid valves include a low-reverse pressure control valve LR-PCV, a second pressure control valve 2ND-PCV, an underdrive pressure control valve UD-PCV and an overdrive pressure control valve OD-PCV.
Further provided is a switch valve SW-V which reduces line pressure in third and fourth speeds of a drive D range such that stress given to an oil pump OP is reduced and a power transmission efficiency of the automatic transmission is improved. The hydraulic control system also includes a first fail-safe valve FSV-1. The first fail-safe valve FSV-1 prevents the simultaneous operation of the low-reverse brake LR-B and the underdrive clutch UD-C, as well as of the low-reverse brake LR-B, underdrive clutch UD-C and overdrive clutch OD-C. There is also provided a second fail-safe valve FSV-2 which prevents the simultaneous operation of the second brake 2ND-B, underdrive clutch UD-C and overdrive clutch OD-C.
As shown in FIG. 5, the solenoid valves are operated to ON and OFF states in different combinations to realize control into the various speeds and shift modes. That is, as described above, the solenoid valves are operated to ON and OFF states such that the supply of hydraulic pressure to the valves is controlled. This, in turn, controls the supply of hydraulic pressure to specific friction elements to control the same, thereby ultimately controlling the multi-stage gear shift mechanism for control into the different shift speeds and modes.
However, in the conventional hydraulic control system described above, a solenoid valve is provided at each of the friction elements, except for the low-reverse clutch REV-C; a pressure control valve is provided at each of the solenoid valves; and two fail-safe valves are required to prevent the simultaneous operation of the friction elements as described above. This large number of elements complicates the hydraulic control system, and increases overall manufacturing costs and the weight of the hydraulic control system.